Dysregulation of lipid metabolism is responsible for pathologies of human diseases including metabolic diseases. Recent advances in lipidomics analysis allow for the targeted and untargeted identification of lipid species and for their quantification in normal and diseased conditions. Herein, this review provides a brief introduction to lipidomics, highlights its application to characterize the lipidome at the cellular and physiological levels under different biological conditions, and discusses the potential for the use of lipidomics in the discovery of biomarkers.

Objective Lipid-laden macrophages or foam cells are characterized by massive cytosolic lipid droplet (LD) deposition containing mostly cholesterol ester (CE) derived from the lipoproteins cleared from the arterial wall. Cholesterol efflux from foam cells is considered to be atheroprotective. Since cholesterol is effluxed as free cholesterol (FC), CE accumulation in LDs may limit FC efflux. Our objective was to identify proteins that regulate cholesterol trafficking through LDs. Approach and results In a proteomic analysis of the LD fraction of RAW 264.7 macrophages we identified an evolutionarily conserved protein with a canonical GXSXG lipase catalytic motif and a predicted α/β-hydrolase fold, the RIKEN cDNA 1110057K04 gene, which we named lipid droplet-associated hydrolase (LDAH). LDAH association to LDs was confirmed by immunoblotting and immunocytochemistry. LDAH was labeled with a probe specific for active serine hydrolases. LDAH showed relatively weak in vitro CE hydrolase activity. However, cholesterol measurements in intact cells supported a significant role of LDAH in CE homeostasis, since LDAH upregulation and downregulation decreased and increased, respectively, intracellular cholesterol and CE in HEK293 cells and RAW 264.7 macrophages. Mutation of the putative nucleophilic serine impaired active hydrolase probe binding, in vitro CE hydrolase activity, and the cholesterol lowering effect in cells, while this mutant still localized to the LD. LDAH upregulation increased CE hydrolysis and cholesterol efflux from macrophages and, interestingly, LDAH is highly expressed in macrophage-rich areas within mouse and human atherosclerotic lesions. Conclusions The data identify a candidate target to promote reverse cholesterol transport from atherosclerotic lesions.

Genetic screens in cultured human cells represent a powerful unbiased strategy to identify cellular pathways that determine drug efficacy, providing critical information for clinical development. We used insertional mutagenesis-based screens in haploid cells to identify genes required for the sensitivity to Lasonolide A (LasA), a macrolide derived from a marine sponge that kills certain types of cancer cells at low-nanomolar concentrations. Our screens converged on a single gene, LDAH , encoding a member of the metabolite serine hydrolase family that is localized on the surface of lipid droplets. Mechanistic studies revealed that LasA accumulates in lipid droplets, where it is cleaved into a toxic metabolite by LDAH. We suggest that selective partitioning of hydrophobic drugs into the oil phase of lipid droplets can influence their activation and eventual toxicity to cells.

BackgroundFoam cells are central to two major pathogenic processes in atherogenesis: cholesterol buildup in arteries and inflammation. The main underlying cause of cholesterol deposition in arteries is hypercholesterolemia. This study aimed to assess, in vivo, whether elevated plasma cholesterol also alters the inflammatory balance of foam cells.Methods and ResultsApolipoprotein E–deficient mice were fed regular mouse chow through the study or were switched to a Western‐type diet (WD) 2 or 14 weeks before death. Consecutive sections of the aortic sinus were used for lesion quantification or to isolate RNA from foam cells by laser‐capture microdissection (LCM) for microarray and quantitative polymerase chain reaction analyses. WD feeding for 2 or 14 weeks significantly increased plasma cholesterol, but the size of atherosclerotic lesions increased only in the 14‐week WD group. Expression of more genes was affected in foam cells of mice under prolonged hypercholesterolemia than in mice fed WD for 2 weeks. However, most transcripts coding for inflammatory mediators remained unchanged in both WD groups. Among the main players in inflammatory or immune responses, chemokine (C‐X‐C motif) ligand 13 was induced in foam cells of mice under WD for 2 weeks. The interferon‐inducible GTPases, guanylate‐binding proteins (GBP)3 and GBP6, were induced in the 14‐week WD group, and other GBP family members were moderately increased.ConclusionsOur results indicate that acceleration of atherosclerosis by hypercholesterolemia is not linked to global changes in the inflammatory balance of foam cells. However, induction of GBPs uncovers a novel family of immune modulators with a potential role in atherogenesis.

Accumulation of atherosclerotic plaques in arterial walls leads to major cardiovascular diseases and stroke. Macrophages/foam cells are central components of atherosclerotic plaques, which populate the arterial wall in order to remove harmful modified low-density lipoprotein (LDL) particles, resulting in the accumulation of lipids, mostly LDL-derived cholesterol ester, in cytosolic lipid droplets (LDs). At present, LDs are recognized as dynamic organelles that govern cellular metabolic processes. LDs consist of an inner core of neutral lipids surrounded by a monolayer of phospholipids and free cholesterol, and contain LD-associated proteins (LDAPs) that regulate LD functions. Foam cells are characterized by an aberrant accumulation of cytosolic LDs, and are considered a hallmark of atherosclerotic lesions through all stages of development. Previous studies have investigated the mechanisms underlying foam cell formation, aiming to discover therapeutic strategies that target foam cells and intervene against atherosclerosis. It is well established that LDAPs have a major role in the pathogenesis of metabolic diseases caused by dysfunction of lipid metabolism, and several studies have linked LDAPs to the development of atherosclerosis. In this review, several foam cell-targeting pathways have been described, with an emphasis on the role of LDAPs in cholesterol mobilization from macrophages. In addition, the potential of LDAPs as therapeutic targets to prevent the progression and/or facilitate the regression of the disease has been discussed.